Sensors that detect the presence or movement of a non metallic object through a flat article, for example through a panel, which is not transparent to optical radiation are usually constructed on the basis of capacitance measuring techniques. Known applications are e.g. capacitive Touchpads such as are known from e.g. DE 103 24 579 A1, capacitive proximity sensors and so-called “stud detectors”.
In the two first mentioned applications, the sensor unit is firmly connected to the panel that is to be penetrated and, as seen from the sensor unit, the object being detected moves behind this panel. Due to the mechanical arrangement thereof, the sensor and the panel have a fixed mutual capacitance which is reflected in the measured value as a constant basic capacitance.
Further applications are sensors which have to be moved over the flat article or the panel in order to locate objects lying behind it such as are known from e.g. EP 0 657 032 B1 and EP 1 740 981 B1. Falling within this category are so-called “beam finders”or “stud detectors”. In general, beam finders are handy devices for do-it-yourself enthusiasts and professionals which are used for detecting beams, posts or pipes or current-carrying wires located behind solid wooden boards or paneling in e.g. prefabricated buildings. For this purpose, the sensor is guided across the wall. It measures the capacitance with respect to the wall by means of an electrode. If a timber beam, a pipe or a current-carrying wire is located within the detection range of the sensor, then this capacitance increase due to the change in the dielectric. This is evaluated accordingly and brought to the user's attention. As long as the sensor is being moved at exactly the same distance from the flat article or the panel, the capacitance between the sensor and the flat article or the panel does not change. It merely enters the test signal or the measured value signal in the form of a constant value as was the case in the first two examples. However, using the example of the stud finder, one can appreciate that, due to the manner of construction of the wall, it is almost impossible in practice to maintain a constant spacing so that the basic capacitance will change substantially in dependence on the spacing. Consequently, the stud finder has also been selected to serve as an exemplary embodiment in the following description of the invention.
In general, the basic capacitance that is formed as a result of the construction of the wall is substantially higher than the increase in capacitance due to an object located behind or within the wall. When the stud finder is being moved over the wall, even tiny amounts of tilting caused by the unevenness of the wall can cause a very large reduction in the capacitance so that an object requiring detection can no longer be recognized. This effect is particularly noticeable in the case of textured plaster, wood chip wall papers or at the joints between strips of wallpaper. Not only does textured plaster lead to tilting of the sensor, but it is usually applied unevenly as well. Since the thickness of such a layer of plaster also has a strong influence on the result of the measurement made by the sensor, the search for the beam or post becomes a matter of pure luck. Moreover, local inhomogeneities in the construction of the wall especially when they are located near the surface i.e. in the proximity of the sensor also enter into the basic capacitance.
In order to provide a better explanation, the effect of the tilting process will now be illustrated on the basis of a conventional prior art sensor. FIG. 11 shows a sensor 1.3 incorporating an electrode 5.12 for the determination of the capacitance by means of an electromagnetic field 5.15. The associated electronic system is not illustrated, it being assumed that its mode of functioning is familiar.
If, in accordance with FIG. 1, the sensor 1.3 is moved from a given position towards a wall 1.1, then the capacitance value C as illustrated in the measured value curve 1.7 alters to higher values until the nearest possible point of proximity to the wall 1.1 is produced. An ideal movement along the wall will not change the measured value as long as a post or a beam 1.2 or the like is not located behind the wall. In order to indicate the presence of a beam or the like, a threshold value 1.6 as an example can be set above the measured value 1.7 in such a way that it will be exceeded due to the resultant increase in the measured value 1.8 which occurs when passing over a beam 1.2.
However, FIG. 2 shows a measured value curve such as often occurs in reality. The wall 1.1 illustrated here has a plaster coating 2.5, irregularities 2.1 and inhomogeneities 2.2. The irregularity can, for example, be a somewhat thicker point in the plaster or a joint in the wallpaper. When the sensor 1.3 encounters the irregularity 2.1, the surface of the sensor is forced slightly away from the wall. As a consequence, the capacitance is reduced, and the measured value curve 1.7 drops accordingly (FIG. 2, 2.3). Since the threshold value 1.6 is not exceeded in this case, this does not at first present a problem.
It does become critical however, if this irregularity lies in the vicinity of the beam 1.2 that is to be located. The result then is that the beam is not recognized (FIG. 2, 2.6). That is to say, the increase in capacitance due to the beam behind the wall is obscured due to the tilting of the sensor on the irregularity 2.1. In this case, the threshold value 1.6 is not exceeded and in consequence the beam is not located. There is a different effect in the case of an inhomogeneity, e.g. a nail. Here, the measured value 1.7 might possibly increase to such an extent that the threshold value 1.6 is exceeded and a spurious “beam” is signaled.
The detected decrease in capacitance of the sensor when it tilts or rises slightly away from the wall in accord with the prior art is illustrated in FIG. 6. It can be clearly perceived that in the state of the art, the largest change in the capacitance curve 6.1 occurs within the first few millimeters of a tilting action or distancing of the sensor from the surface (FIG. 6, D).
From DE 10 2005 031 607 A1, there is known a device for converting the capacitive change of signal of a differential capacitor which is used in acceleration sensors into a digital signal. A sigma delta modulator is used for this purpose. The principle of sigma delta modulation is based on a rough measurement of a signal by means of a quantizer. The measuring error arising thereby is integrated and continually compensated for by means of a negative feedback arrangement. In dependence on the type of conversion process being undertaken, the individual blocks of the sigma delta modulator are implemented in a digital or analogue manner. The differential capacitor is integrated into the negative feedback path and the reference feedback structure. A change of capacitance can thus be converted directly from an analogue value into a digital signal. Due to the integration of the differential capacitor, the output signal of the converter in the form of a binary stream is to a first approximation only dependent on the deflection of the seismic mass of the differential-capacitor. Differing reference voltages, which are selectable with particular amplitudes in a specific temporal pattern, are applied to the electrodes of the differential-capacitor. Different ranges of values and resolutions of the signal that is to be digitized can be represented by the differently selected reference voltages. A change of polarity of the input signal is realized by a suitably selected sequence of the reference voltages. In addition, an adjusting force can be exerted on the moveable electrode for the purposes of performing a self checking function by means of the sequence of reference voltages and a suitable clock-timing pattern for connecting the reference voltages to the electrodes. Averaged over time, potential equality of the electrodes is achieved by the choice of the reference voltages. Thereby, electrical charging of the seismic mass and the change of the output signal arising therefrom are prevented. There is however no oppositely directed regulation of a sensor electrode and of a further electrode surrounding the sensor electrode.
DE 198 43 749 A1 depicts a method and a circuit arrangement for evaluating small changes of capacitance in a capacitive proximity switch. To this end, use is made of a bridge circuit in which a reactance in the form of a capacitor is located in each arm of the bridge. The two bridge arm voltages are rectified separately in accord with the respective bridge arm, whereafter the diagonal bridge voltage is evaluated in the form of a DC voltage which varies in correspondence with the change in capacitance of the capacitor. The proximity switch consists of a multi-layer printed circuit board comprising two electrically insulating layers between which there is a metallic intermediate layer that serves as the first surface of a capacitor. A flat coating serving as a probe which forms the second surface of the capacitor is applied to one of the two layers. A metal surface is arranged such as to be moveable relative to the probe and forms a second variable capacitor therewith. Increased immunity to noise as well as increased temperature stability are achieved by means of this circuit arrangement. For this purpose, the circuit is adjusted by means of a variable capacitor in such a manner that the differential voltage is equal to zero at the time point when the proximity switch operates since it is sufficient then to merely evaluate the prefix sign.
In U.S. Pat. No. 7,148,704 B2, there is depicted a capacitive arrangement for establishing the position of an object namely, that of a finger on a Touchpad. The sensor used for this purpose comprises two measuring channels which are each connected to a respective electrode. The channels are operated in synchronism, whereby each channel gives a non-linear response to a capacitive effect produced by the finger. These respective output signals are combined linearly in order to supply position signals which change linearly with the position of the finger, whence the sensor works as a ratiometrical sensor.
U.S. Pat. No. 5,585,733 depicts a device and a method for measuring the change in capacitance of a capacitive sensor. To this end, means are provided for applying a constant electric current to an electrode and means are also provided for producing a first series of clock pulses. The voltage of the capacitor is compared with a reference voltage, whereby a signal is produced if the voltage of the capacitor exceeds the reference voltage. The capacitive sensor is used for measuring the change in a dimension of an article having variable dimensions such as a telescopic device for example. The capacitor is formed by two electrically conductive layers which surround dielectric sleeves of a piston. The capacitance of the capacitor is changed by the movement of the piston and thus of the sleeves. This change is detected and evaluated by the control system, whereby the change in the position of the piston is detected.
DE 39 42 159 A1 depicts a circuit arrangement for the processing of sensor signals which are detected by means of a capacitive sensor. The sensor comprises a measuring capacitance which is arranged to be affected by the physical variable requiring detection and a reference capacitor which exhibits a reference capacitance and supplies a measuring effect that is dependent on the measuring capacitance and the reference capacitance. First electrodes of the capacitors of the sensor are kept at a fixed potential, whilst, for the purposes of carrying out a charge transfer process, the second electrodes thereof are connected to a first input of an input operational amplifier the second input of which is at a reference potential. The reversal of charge on the capacitors of the sensor that is necessary for producing the trans-ported charge is effected by switching over the reference potential of the input operational amplifier. Consequently, the two inputs of the operational amplifier are virtually at substantially the same potential. Thus, due to the change-over of the reference potential of the input operational amplifier, the potential of the capacitor electrode connected to the other input is also changed in the same way on each occasion. Consequently, the necessary changes in voltage on the capacitor electrodes connected to the signal line are made so that only they are active. The other electrodes are then inactive and are at any arbitrary potential, for example, at the potential of the housing for the sensor.